Variable output linear accelerator



Jan. 5, 1960 E. L.. GlNz-roN 2,920,228

VARIABLE OUTPUT LINEAR ACCELERATOR Filed Dec. 13, 1954 fl g Zi/ il United States arent VARIABLE OUTPUT LINEAR ACCELERATOR Edward L. Ginzton, Los Altos, Calif., assignor to The Board of Trustees of The Leland Stanford Jr. Uniu versity, Stanford University, Stanford, Calif.

Application December 13, 1954, Serial No. 474,716

6 Claims. (Cl. S15- 3.6)

This invention relates to particle accelerators of the linear type, the output whereof is continuously variable with respect to the energy imparted to the particles, from zero up to some maximum capacity. Particularly, it relates to electron accelerators.

Developments in the design of electron accelerators have made the use of these devices practical in many applications where high-energy electron streams are required. One example of the use of devices of this kind is as a source of high-energy electrons for developing X-rays for deep therapy orY for commercial use in testing for flaws in metal, such as forgings, castings, or the like. Other uses for high energy electron streams are developing constantly. This invention is not particularly concerned with utilization made of the high-energy stream after it has been developed, but merely with the apparatus used for imparting energy to the electrons, irrespective of the duty ultimately accomplished by the electron stream.

One difliculty with the employment of linear electron accelerators has been their inflexibility. In order to give the electrons their high energy it is necessary that .they travel with nearly the speed of light. The electronsare injected into a wave guide so designed that the phase velocity of the Waves therein is substantially equal to the velocity of waves in free space. The injected electrons which are to be accelerated ride the crests ofthe waves traveling down the guide, gaining energy as they go. If they are to ride the crests of the waves through a guide of appreciable length, `they mustbe brought up to something approximating their ultimate speed within a relatively short distance from the point at which they are injected. If suflicient energy is fed into the accelerator to give the electrons the necessary initial velocity the `resultant waves will be Vpropagated down the wave guide to give the particles their iinal -energy which can be varied only through a relatively narrow range.

AFor many applications the `resulting iniiexibility lis a definite disadvantage. In industrial .radiography lit is desirable'to use X-rays of the minimum .penetration ,possible'which'will penetrate the thickness of material to be tested. `In X-ray therapeusis it maybe desirable to treat a cancer located in the middle of the body with X-rays originating from the impact o'f electrons having energies as high as thirty million electron volts, whereas to treat aneoplasm near .the surface of the body a million electron volts or less may be ample. ln the use of vsuch a device for food sterilization high-energy electrons may be desirable for food in certain types .of containers, whereas'a much smaller energy is necessary to treat food in Ycontainers of other varieties. "For researchinnuclear physics itis obvious that a wide range of electronenergies available increases the utility of theelectron accelerator .asa research tool. I

The .primary purpose of the present invention is gto provide a particle acceleratorof thelinear 4type wherein thellenergy ofgthe particles may be .varied to any desired f'ice degree up to the maximum for which the device is b uilt. Contributory to this broad purpose, among the objects of the invention are to provide a variable-energy electron source, which, for the energy available, is relatively light and compact, to provide a particle accelerator wherein the energy of the emergent particles may be varied continuously, rather than in steps, so that an exact 'adjustment to a desired energy value may be obtained; to provide a variable-output-energy linear accelerator which is applicable to commercial and medical as well as laboratory use; to provide an accelerator which can be matched to available microwave power sources, such as magnetrons and klystrons; and to provide an adjustable linear accelerator of the class described which may be constructed by known and tested techniques, and the individual elements whereof, as well as the assembled device, are sturdy, reliable, and relatively light and inexpensive as `compared with other devices capable of delivering particles of comparable energy.

Broadly considered the electron accelerator of this invention comprises two sections of loaded wave guide, each section, individually, being of conventional type. The two sections are mounted in linear alinement, and on one, which may be termed the input section, there is mounted an electron gun positioned to direct a beam kof electrons longitudinally through both sections, emerging at the end of the output section, that most distant from the electron gun. Each of the sections is provided at its input end (i.e., the end Vnearer the electron gun) with coupling means through which the section may be supplied with radio-frequency power for accelerating electrons injected into the structure by the electron gun, and means are provided for supplying to the respective coupling elements radio-frequency waves from a single source, capable of accelerating the injected electrons, but of variable relationship, in phase, amplitude, or both, as between the waves supplied to the two sections. Means may be provided for absorbing the radio-frequency waves transmitted by the sections, at the output ends if so desired, although as will be shown hereinafter, this is not a necessary feature of lthe invention.

The following detailed description of certain preferred embodiments of the invention will explain the above more fully. In these descriptions reference will be made to the accompanying drawings wherein:

Fig. l is a schematic diagram of an accelerator in accordance with the present invention, certain conventional elements being illustrated in block form;

Fig. 2 is a similar but less detailed diagram of a modified form of the invention; and

Fig. 3 is a diagram illustrating an alternative arrangement for supplying radio frequency energy to an accelerator of the type illustrated in Fig. l.

In the first figure there is shown diagrammatically a accelerator vcomprising two sections, numbered respectively 1 and 3, of loaded wave guides, i.e., of a wave guide so arranged that the phase velocity of the waves transmitted thereby is slowed down to substantially the free-space speed of light. Various types of loaded lwave guides for accomplishing this are known in the art; that v illustrated is a guide ofthe so-called disc loaded type,

Each of the sections is provided at its input endrwth a coupling cavity, designated as 9 and 11 respectively,Y

at which radio frequency energy can be introduced into the guide. These cavities are circular in the form here illustrated and they are closed at their input ends, with diaphragrns pierced by small holes of only sufficient diameter to admit an electron beam and insufcient to permit the transfer of appreciable amounts of energy between the sections, the degree of coupling afforded ythrough these apertures being negligible. v

There is also shown inthis figure, as a desirable a1- though not a necessary element, a similar-coupler 13* at the output end of thewave guide section 3. lf the entire structure is to be evacuated, instead of being contained in an over-all evacuated envelope, the output end of the device where the coupler 13 is shown, must be closed, vacuum-tight whether the coupler is used or not.v Such closure may be by means of a Lenard window 15 which permits the emergenceY of the accelerated electrons. This would be the preferred method if the device is'to be used for sterilization of food products by direct bombardment of electrons, and hence is that shown in the present application, wherein it is the accelerator itself and not its ancillary equipment which is of interest.

At the input end of section 1 a conventional electron gun 17 is mounted. The gun comprises a cathode 19 and one or more anodes 21 for giving the electrons an initial acceleration in a D.C.V field prior to their injection through the aperture in the coupler 9. As various types of electron guns are well known in the art'and have been used in the past for the same purpose as that here shown the gun is not described in detail.

The coupler 9 is shown as receiving energy from au oscillator 23 through a wave guide 25, feeding into the coupler through a matching iris 27. The oscillator 23 v is conveniently a magnetron or a klystron, capable of generating microwaves in the frequency range. to which the particular structure used is adapted. Any v other generator of oscillations within this range, which is capable of delivering high power, is suitable, the two mentioned being those which have received the greatest development up to the present time and are therefore best known in the art.

Oscillator 23 also supplies waves of the same frequency as those supplied to the coupler 9, through a wave guide 29, to either an attenuator 31, a phase-shifter 33, or, preferably both, and thence, through wave guide 29' to coupler 11. If the coupler 13 is used to terminate the structure, the latter coupler connects through a wave guide 35 to a load 37, which absorbs the radio frequency power transmitted through the guides to the output end of the tube and prevents reections which would set up material standing waves in the output section 3.

The phase shifter 33 may, for example, be of the slotted Wave guide type illustrated in Microwave Transmission Circuits (Ragan, vol. 9, Radiation Laboratories Series, McGraW-Hill, 1948, page 513), or it maybe of the dielectric type illustrated in the s ame work at pagey 514. One that has been used in practice is the dielectric type, utilizing Mycalex as the dielectric. The attentuator 31 can be quite similar to the phase-shifter except for the fact that resistive material which will dissipate the energy is used in place of the dielectric. Various means are known for adjusting the attenuationby introducing the losser element of the attenuator a greater or less distance into the wave guide, or by altering its position within the guide between points where it is in practically a zero electric materials, are illustrated and ldescribed in Technique of Microwave Measurements, vol. 11 of the Radiation Laboratories Series above cited, at page 748 and follow-v ing.

`Eitherthe attenuator or the phase-shifter may be usedvv 29' is in phase with the bunched electrons, the emergent 4 sary, and this gives a much more flexible device than the attenuator alone. In general, however, it is difficult to make the effective length of the guides 29 and 29 have an exact phase relation to the effective electrical length of the guide 25 plus the section 1 of the loaded guide, and therefore, even if the attenuator is used, the phase adjuster is desirable. On the other hand, there always exist some electrons which are out of step with the actual crests of the waves as they enter the section 3 of the loaded guide. If phase rotation alone is used to adjust the output power, it is possible that a phasing of the supply to the section 3 which will decelerate the major group of electrons entering this section may be in proper phase to accelerate the out-of-step electrons and yield an output beam which is less homogeneous than is desired and which may Vcomprise groups of electrons which are of much higher energy than is wished. It is for this reason that maximum flexibility o-f adjustment dictates the use of both adjusting mechanisms.

In the operation of the device as shown in Fig. 1, it is preferable that pulsed oscillations be used from a source` 23. The spacing of the loading discs 7 which slow the phase velocity of the waves traveling down the guide is preferably such in the initial portion of the guide, close to the electron gun 17, that the phase velocity is materiallyy lower than that of light, increasing with increased velocity of the electrons, until in the major portion of thedevice the phase velocity of the waves transmitted by the guide section is very nearly that of free space. If the spacing` of the loading discs is properly arranged, certain of the electrons injected by the gun will be subjected to accelerating fields during their entire path through the section l of the wave guide. Certain` others will be subjected to decelerating fields which slow them somewhat in the early stages of their passage, and there is a tendency ofthe electrons to bunch in the accelerating field. The major part ofthe electrons in the beam, as it enters the coupler 11, will therefore be ltraveling at maximum velocity and have maximum energy.

What happens to the electron beam after it enters the guide section 3 depends on both the phase and the magnitudetof the microwave power injected at the coupler 11 through the guide 29. Assuming either complete attenuation of the power normally supplied through guide'29',y or energy injected 90 degrees out of phase with the bunch.

ing of the electrons' delivered into the second section of the coupler, the electronsissuing from the Lenard'window 15 will neither gain nor lose energy `as they pass through the second section of the accelerator and will emerge with one-half the energy of which the device is capable. y If the wavesv in the second section are 180 de' grees out of phase with what they would be if they were transmitted directly between the section, the emergent electrons will be reduced to practically zero energy. If, however, the phase of the energy fed to the second section is the same as it would be if the two sections were directly coupled, i.e.,'if the energy supplied through guide n electrons will have substantially double the energy they field or in maximum field. Such attenuators, of different would have had'under the conditionsV first considered, and the device willrdeliver'its full power output; Y If the electrons of the beam are fully ,bunched as they enter the section 3 of the wave guide, a shift of phase in the RF energy supplied to the latter will displace the crests of the waves with respect to the bunches so that they no j longer ride the crests, but are subjected to accelerative v fields of lower than maximum value, and hence, less. energy is added to the beam.A Decreasing the amplitude To subtracty energy from the beam, however, the phaseV must be shifted more` of the crests has theV same effect.

than degrees. Less than complete subtraction ofthe energy in vthe beam can be accomplished either vby phasereversal and attenuation or by phase rotation between 90 degrees and degrees or both. It has been mentioned that the waves supplied from theV oscillator Y2? are preferably pulsed. This is particularly important where a magnetron source is used, such sources being very susceptible to changes in effective load impedance such as may be caused by standing waves in the accelerator. The same loading which slows the phase velocity, however, also slows the envelope or energy ve- `locity of waves injected into the input end of the device.

The degree of loading which has been used in practice slows the energy velocity down to approximately 1/100 of the Speed of light. If it be assumed that the envelope or energy velocity is so chosen that it takes (say) l microsecond for waves supplied to the coupler 9 to reach the output end of the'iirst section, it will take another microsecond for a reiiected wave to return to the coupler. Until the reflected wave does so return it can have no effect on the apparent yimpedance seen by the oscillator. If the oscillator pulses, therefore, do not exceed 2 microseconds in length, under the conditions assumed, the standing waves will have no effect upon the radio frequency source, and hence they can be neglected from this aspect.

The reected waves are traveling in the opposite direction from the electrons in the stream, and since the latter are traveling substantially at the speed of light the reflected waves -are traveling at double that speed yin comparison to the electron stream. The fields of the reected waves therefore alternately accelerate and decelerate the electrons in the stream, and, in effect, cancel out, so that from this aspect also they are of no importance.

' A modification of the invention is shown in Fig. 2. In

this case, as in the embodiment shown in the first figure, two loaded wave guide sections 1 and 3' are employed. The arrangements at the input and output ends of the device may be identical with those of the form shown in Fig. l, and the'various parts are therefore indicated by the same reference characters, distinguished by accents, and will not again be either described or shown in detail. In this form of invention, however, an additional coupler l41'is provided at the output end of the wave guide section 1', and the iirst section, instead of feeding directly into the guide section 3', is separated from the latter by a drift tube 43 through which the electrons pass, without acceleration, between the coupler 41 and the input coupler 11' which excites wave guide section 3. An unloaded wave guide 45, matched in impedance to the coupler 41, withdraws radio frequency energy received at the output end of section 1', connecting to attenuator 31 and phase shifter 33', and thence connecting back into coupler 11'.

It will be seen that if wave guide `4.5 is properly matched to coupler 41 and the loaded wave guide section l', the power reaching that point will be passed through the attenuator and phase shifter instead of being reflected and may be used to excite the guide section 3'. The functions of the attenuator 31' and phase shifter 33' are exactly the same as in the embodiment of the invention first shown. There will, of course, be attenuation in the radio frequency power reaching the coupler y41, and hence the coupler 11', even though the adjustment of attenuator 31 is for a minimum attenuation. For this reason the maximum energy that can be either added to o-r subtracted from the electrons in the accelerator section 3' will not be as great as in the case of the iirst embodiment shown, so that the minimum energy available in the output electron stream will be greater and the maximum energy less than in the first form of the device. In certain applications, however, this embodiment may be preferable.

Fig. 3 indicates a modified arrangement fo-r supplying the radio frequency driving power to an accelerator of the type shown in Fig. l. in this arrangement a relatively low power radio frequency oscillator 51 is used as a driver. It is coupled, through unloaded wave guides 53 and 55, with a radio frequency amplifier 56 which connects into coupler 9 in the same manner as the oscillator 23 of Fig. l. Wave guide 53 also connects to a variable attenuator '57 and phase shifter 59, and the output of the lattersupplies a second radio frequency amplifier 6'1. Amplifier 61 supplies coupler 11.

The arrangement of Fig. 3 possesses the advantages that the attenuator 57 and phase shifter 59 need not handle power at such high levels as attenuator 3l and phase shifter v33. Amplifiers 156 and -61 can, individually, be of lower power than Aoscillator 23, and this may be of advantage, vparticularly if high power tubes are not available. There is, of course, a saving in operating the attenuator and phase shifter at low level.

y It should be evident that many of the features that are described herein are illustrative only and that other modiiications are possible besides those which have been specifically described. Details of the invention will, of course, be modified in accordancer with the use to which the y'accelerator is put, whether for laboratory use, electron bombardment for the preservation of foods, or as a generator of X-rays. Such varying usages will require different ancillary equipment and the use of such equipment is contemplated. The detailed descriptions in this specification are therefore not intended to be restrictive, all intended limitations being expressed in the claims which follow.

What is claimed is as follows:

l. A variable-energy-output particle accelerator comprising a loaded wave guide divided into a first and a second linearly alined section, said first and second sections being connected together in communicating relationship with electrically decoupling means therebetween, an electron gun positioned to direct a stream of electrons longitudinally through both of said wave guide sections successively, separate means for coupling radio frequency energy into the input ends of the respective sections, a source of radio frequency energy, connections for supplying said energy to both of said coupling means so that electromagnetic waves are transmitted through the lengths of both of said sections, and adjustable means for varying the phase-amplitude relationship between the energy supplied to the first of said sections and that supplied to the second thereof, said first section being constructed to transmit said vvaves in electron-accelerating relationship to said stream and said second section being constructed to transmit said waves in variable electron-accelerating or electron-decelerating relationship to lsaid stream, selectively, depending upon the adjustment of said adjustable means.

2. A variable-energy-output particle accelerator comprising a loaded wave guide including an input section and an output section in linear alinement and connected together in communicating relationship with electrically decoupling means therebetween, each of said sections being constructed to transmit electromagnetic waves with phase velocities approximately equal to the velocity of light, an electron gun mounted to direct a beam of electrons longitudinally through both of said wave guide sections, means for coupling external circuits to said wave guide at the input end of each of said sections, means for supplying radio-frequency power at the same frequency to the coupling means at the input ends of both sections so that electromagnetic waves are transmitted through the lengths of both of said sections, and adjustable means for varying the phase-amplitude relationship between the power supplied to the input section and that supplied to the input end of said output section, said input section being constructed to transmit said waves in electron-accelerating relationship to said beam and said output section being constructed to transmit said waves in variable electron-accelerating or electron-decelerating relationship to said beam, selectively, depending upon the adjustment of said adjustable means.

3` A variable-energy-output particle accelerator structure comprising two loaded wave guide sections and a drift tube connecting said sections and in alinement therewith providing a communication between said sections 'gun mounted to direct an electron beam through said sections and said drift tube, means for supplying radio frequency power to the structure at a point adjacent'to said electron gun, a loop circuit for radio frequency power connecting said sections around said drift tube so that electromagnetic waves are transmitted through the lengths of both sections, a load circuit for absorbing radio frequency power coupled to said structure at the end remote from said electron gun, and adjustable means in said loop circuit for varying the radio frequency power transferred thereby between said sections, the one nearer said gun of said sections being constructed to transmit said waves in electron-accelerating relationship to said beam and the other of said sections being constructed to transmit said waves in variable electron-accelerating or electron-decelerating relationship to said beam, selectively, depending upon the adjustment of said adjustable means.

4. A variable-energy-output particle accelerator structure comprising two loaded wave guide sections-and a drift tube connected in alignment between said sections and providing electrically decoupling communication between said sections, an electron gun mounted to direct an electron beam through said sections and said drift tube, means for supplying radio frequency power to the structure at a point adjacent to said electron gun, a loop circuit for radio frequency power connecting said sections around said drift tube so that electromagnetic waves are transmitted through the lengths of both sections, a load circuit for absorbing radio frequency'power coupled to said structure at the end remote from said electron gun and a variable phase-shifter in said loop circuit, the one nearer said gun of said sections being constructed to transmit said waves in electron-accelerating relationship to said beam and the other of said sections being constructed to transmit said waves in variable electron-accelerating or electron-decelerating relationship to said beam, selectively, depending upon the adjustment of said variable phase-shifter.

5. A variable-energy-output particle accelerator structure comprising two loaded wave guide sections and a drift tube connecting said sections and in alinement therewith said drift tube providing communication between said sections of an electrically decoupling character for the passage of particles therethrough, an electron gun mounted to direct an electron beam through said sections and said drift tube, means for supplying radio frequency power to the structure at a point adjacent to said electron gun, a loop circuit for radio frequency power connecting said sections around saidV drift tubeso that electromag- Ynetic waves are transmitted through the lengthsof both sections, a load circuit for absorbing radio frequency power coupled to said structure at the end remote from said electron gun, and an adjustable attenuator of radio 'frequency power in said loop circuit, the one nearer said gun of said sections being constructed to transmit said waves in electron-accelerating relationship to said beam and the other of said sections being constructed to transmit said Waves in variable electron-accelerating relationship to said beam, depending upon the adjustment of said attenuator. Y Y

6. A variable-energy-output'particle accelerator comprising a structure of two alinedloaded wave guide sections connected togetherl in communicating relationship with electricallyV decoupling means therebetween, an electron gun positioned to direct a beam of electrons longitudinally through both of said sections, coupling means for supplying radio frequency energy to said structure adjacent to said electron gun and at the junction vof said sections so that electromagnetic waves are transmitted through the lengthsk of both sections, a common source of radio frequency energy, and adjustable means for supplying energy from said source to said respective coupling means in differing phases land amplitudes, the one nearersaid gunk of said sections being constructed to transmit said waves in electron-accelerating relationship to said beam and the other of said sections being constructed to transmit said waves in variable lelectron-accelerating or electron-decelerating relationship to said beam, selectively, depending upon the'adjustment of said adjustable means.

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